Quantitative evaluation of a portal film contrast enhancement technique.

A study was conducted to evaluate the subjective improvement in portal film image quality resulting from the contact copy contrast enhancement technique which was introduced six years ago. Five observers were asked to identify and orient polyvinyl chloride cylinder images on both original and contrast-enhanced portal films taken with a 10-MeV linear accelerator. Fixed reviewing periods (T) were alloted of 20, 40, and 60 s as well as unlimited viewing time in order to increase the clinical relevance of this comparison. A scoring system and a probability representation were used to compare the original and enhanced films as a function of T. The results show a substantial increase in object detectability for the enhanced films at the short viewing times (T = 20, 40, and 60 s). For longer times (T greater than or equal to 80 s) the object detectability for enhanced and original films is not statistically different.